This invention relates to driver circuits for a plurality of gas discharge lamps.
In a conventional prior art driver circuit for a plurality of gas discharge lamps a single set of output terminals is provided and the lamps are connected in series between the single set of output terminals. In use of such a driver circuit, if one of the lamps fails or is removed, drive is no longer applied to the remaining lamp or lamps.
From U.S. Pat. No. 4,503,361 it is known to connect a respective capacitor in parallel with each of two gas discharge lamps. The function of each capacitor is to change the resonant frequency and power factor of the driver circuit when effectively brought into circuit if its respective lamp is removed. However, the purpose of the change is to protect transistor components of the drive circuit from high voltages rather than to ensure continued drive to the other lamp: in order to ensure that the lamps are driven independently the two lamps are driven from separate transformers.
From U.S. Pat. No. 4,525,649 it is known to connect the filaments of a plurality of gas discharge lamps in series, with the filaments of each lamp being connected via a respective capacitance. The capacitances are chosen so as to present (i) a lower impedance to drive current before the lamps become lit so as to allow the filaments to be heated, and (ii) a higher impedance to drive current after the lamps become lit so as to minimize power consumption subsequent to ignition of the lamps.